The present disclosure is related to the field of radiographic imaging, radiotherapy and analysis thereof. More specifically, the present disclosure is related to water-equivalent phantoms for calibration and/or quality assurance purposes in radiology.
X-rays and other radiological techniques are important diagnostic and/or therapeutic tools. However, the measurement of absorbed doses within and around radiated body tissue necessitates calibration of these radiological devices with phantoms constructed of carefully selected materials. The use of such phantoms permit the determination of absorbed doses and correction factors for use radiography or radiotherapy when information on the energy and nature of charged particles at the point of interest is incomplete or fragmentary.
In such calibration or quality assurance methods or processes, water equivalent phantoms provide an important role. Radiodensity refers to the relative ability of a material to absorb or block passage of electromagnetic radiation, particularly X-rays. Radiodensity is often described according to the Hounsfield scale measured in the Hounsfield unit (HU), which is a common unit for CT number. The Hounsfield unit scale is a linear transformation of linear attenuation coefficient measurements where the radiodensity of distilled miter at standard pressure and temperature (STP) is defined as zero HI the radiodensity of air at standard temperature and pressure (STP) is defined as −1,000 HU. The radiodensity of a material such as bone or calcification may be on the order of 1,000 HU. In order to calibrate a radiographic system to the Hounsfield unit scale, a comparison must be made to distilled water. Imaging a water itself presents the natural problem of containing the water for imaging and the tendency of water to transmit motion, noise, or other vibration, resulting in movement of the water to be imaged. Therefore, a solid material that exhibits the radiographic properties of water is desired for calibration and quality control. Solid material water-equivalent phantoms have been available for approximately 30 years. One challenge in the construction of water-equivalent solid materials is the inherent requirement that the water-equivalent materials have a chemical composition largely different than that of distilled water, yet must be embodied in a material that is solid at standard temperature and pressure. Such material is typically an epoxy, acrylic, or polyethylene base, which is modified by other elements to achieve a desired elemental composition, physical density, effective atomic number, electron density, and radiodensity, such that the attenuating and scattering characteristics closely resemble that of water.
The International Commission on Radiation Units and Measurements (ICRU) in its Report 44 entitled “Tissue Substitutes in Radiation Dosimetry and Measurement” provides approximate elemental, radiographic, and other physical properties for average body tissues, including water. This report, states that a water-equivalent solid phantom must not introduce more than 1% uncertainty to the absorbed dose. If total uncertainty is more than 1%, appropriate correction factors are required to be applied. Therefore, a water-equivalent solid phantom with radiographic properties within 1% that of distilled water is desirable in the field to avoid such requirement for correction factors.
A typical radiation therapy process begins by scanning patient using computed tomography (CT). The resulting three-dimensional CT image data is used in the treatment plan to calculate patient inhomogeneities. To perform true quality assurance (QA) it is ideal to simulate the entire treatment process. Simulation of the entire treatment processes requires scanning a phantom using the CT and then using the same phantom for radiotherapy measurements. For example to perform patient-specific intensity modulated radiation therapy (IMRT) verification measurements, the patient fluence is transferred to the water-equivalent solid phantom and a forward dose calculation is performed. The calculated dose is then compared to the point dose and to the dose distribution measured in the phantom. In this process, it is desirable that the phantom characteristics match attenuation and absorption properties in the diagnostic range (5 keV-150 keV) and therapeutic radiation energies (greater than 1 MeV).
The inventor has recognized that a need exists for new water-equivalent solid phantoms that exhibit a high degree of accuracy to natural water over the entire range of diagnostic and therapeutic energies.